1. Field of the Invention
The present invention relates to a straddle-type vehicle typically referred to as ATV (all terrain vehicle).
2. Description of the Related Art
This sort of vehicle exhibits excellent drivability not only in on-road driving, but also under severe driving conditions typically in off-road driving or at the waterside, showing a high level of mobility. The basic configuration is typically such as having four wheels on the front and rear sides of a chassis frame, a steering device between the front wheels, an engine mounted on the chassis frame, and a fuel tank, a seat and so forth placed thereabove.
Operational stability during driving is a matter of great importance particularly for so-called performance ATV oriented to racing, so that the engine mass is necessarily centralized in order to fulfill the requirement. A specific example of conventional all terrain vehicle will be outlined. Referring to FIGS. 9A and 9B, an engine 1 is mounted on a chassis frame 2 as being located at the center of the vehicle, front wheels 3 are disposed at the front portion of the chassis frame 2, and rear wheels 4 are disposed at the rear portion of the chassis frame 2.
In an exhaust system in this example, an exhaust pipe 5 connected to an engine cylinder on the front side of the engine 1 is once projected towards the front, then routed through one side of the engine 1 (on the right side in the illustrated example) in the tread-wise direction, and extended towards the rear of the vehicle. The exhaust pipe 5 is further extended to a muffler 7 disposed on the rear side of an air cleaner 6, and connected thereto. The muffler 7 is mounted aside a seat rail 9 supporting a seat 8.
The muffler 7 is a heavy matter in the conventional all terrain vehicle, causative of deflection of the center of gravity towards the side where the muffler 7 is disposed (right side in this example), and this largely affects the operational stability of the vehicle. In particular, the lateral balance affects the straight drivability, cornering performance, sliding control performance, grip feeling and so forth. Such deflection in disposition of the muffler 7 from the center of gravity of the vehicle also increases moments in the yaw direction and the roll direction, and affects the motion performance, steering, anti-roll property and so forth.
Also twisting force is exerted on the seat rail 9 supporting the muffler 7, and this raises a need of high rigidity and strength of the seat rail 9 in order withstand the twisting force. The seat rail 9 is sometimes formed as being curved so as to ensure a necessary gap or clearance with respect to the muffler 7 and the exhaust pipe 5, or may tend to deform because the distance between the individual components in the vertical direction is limited. Reinforcement as a countermeasure therefor results in increase in the weight, and also warping and increase in weight of the seat rail 9 adversely affect the operational stability.
Furthermore, for vehicles having a large cushion stroke of the rear wheels 4, it is necessary to ensure a sufficient gap with respect to a rear wheel brake caliper. In order to ensure the gap, it is necessary to dispose the muffler 7 and so forth in an outer portion or in the side portion, but such positional relation unless otherwise being modified inevitably raises a disadvantage in terms of the operational stability.
Japanese Utility Model Application Laid-Open No. S62-11029 discloses this sort of all terrain vehicle having a muffler disposed nearly at the center position between the left and right rear wheels.
Considering the above-described situation, the present invention is aimed at providing a straddle-type all terrain vehicle capable of effectively improving, in particular, the operational stability, the engine performance and the like.